The invention relates generally to a system for analyzing a sputtered or evaporated material deposition process onto a surface of an object, and particularly to such a system for aluminizing the inside surface of a cathode-ray tube (CRT) faceplate panel disposed on an exhaust cart and for determining the aluminum layer thickness profile and exhaust cart operating performance.
An image is produced on the inside surface of a cathode-ray tube (CRT), or picture tube for a color television set, by scanning a phosphor coating on the inside surface of the tube with an electron beam. The impact of the electrons on the phosphor materials causes the phosphors to luminesce and emit the light necessary to produce the image. In order to attract electrons to the phosphor screen it is necessary to place a voltage on the screen. This is accomplished by evaporating a thin layer of conductive material, such as aluminum, over the phosphor coating on the screen. The scanning electron beams which generate the image must penetrate the aluminum layer in order to cause the phosphors to luminesce. For this reason, it is important that the thickness of the aluminum coating across the screen be fairly uniform in order to prevent variations in the light output of the tube.
Typically, the aluminum is distributed onto the phosphor screen by placing the faceplate panel carrying the screen onto an exhaust cart which includes a vacuum pump and a source of aluminum. The aluminum is provided in one or more heaters or sputter sources which cause the aluminum to coat the screen. Typically, when evaporating a layer of material on a large surface, the thickness distribution of the coating material is a function of the number of evaporation sources, the location of the sources with respect to the surface being coated, vacuum conditions within the exhaust cart, and the size of the panel being coated. In an evaporation system the evaporation material is replenished for each evaporation. Therefore, it is desirable to have the fewest number of sources while still obtaining an acceptable distribution on the surface. A single source is not practical because experiments have shown that it is not effective for large rectangular surfaces. Four sources usually suffice if the sources are properly positioned. However, this is disadvantageous because the number of sources increases the needed maintenance, the functions to be performed by the operator and the expense of the operation. Two sources are preferred, and an apparatus for controlling the distribution of evaporated aluminum from two sources onto a screen formed on the inside surface of a panel is described in my copending U.S. Patent Application, Ser. No. 132,516 filed on Dec. 14, 1987.
In some prior art evaporation systems the ratio of the thickest portion of the coated film to the thinnest portion of the film is between 4:1 and 5:1. Thickness ratios of this order are unacceptable for most large CRT's, and particularly for color picture tubes, because changes of this order in the thickness of the evaporated layer cause too great of a variation in the light output across the tube. The apparatus described in U.S. Patent Application Ser. No. 132,516 usually provides a controlled distribution of evaporated material within acceptable limits; however, there is a need to identify, on line, panels having coatings that are too thick or too thin because of either operator error or exhaust cart malfunction.